Three Solutions for Non-conforming FAG Bearing Rings

Three Solutions for Non-conforming Imported Bearing Rings

 I. Centre Offset Method

1. Rectification of the inner and outer diameters of imported bearing rings

First, accurately measure the dimensions at each defective area (asymmetrical type) on the part, calculate the minimum grinding allowance (M) required to achieve the finished dimensional tolerance, and then apply a centre offset during grinding (offset in the direction that increases the grinding allowance at the defective area). The maximum offset is M/2, meaning the grinding allowance is relatively increased at the defective area, with focused grinding applied there.

2. Correction of the inner and outer raceways of the bearing rings

The design reference for the grinding of imported bearing rings is the inner or outer diameter; that is, the inner (or outer) diameter is ground first, followed by the raceways. Taking the inner ring as an example, if there is an asymmetry defect in the inner raceway, the inner diameter of the part must be accurately measured, and the maximum grinding allowance M, as well as the maximum grinding allowance M1 at the defect location on the inner raceway, must be calculated. When M1 < M, the inner ring centre must be offset during the grinding of the inner diameter; the maximum offset is M1/2. This means that during the machining of the inner diameter, the grinding allowance at the defect location on the inner raceway is first increased relatively, and subsequently, when the inner raceway is ground, the defect area receives intensive grinding, thereby salvaging the defective part. When M1 ≥ M, normal grinding will not remove the defect; depending on the situation, the method described in Section 1.1 may be employed to rectify the issue.

        Grinding using the centre offset method is carried out whilst ensuring the part’s hardness and carburisation depth. The total offset cannot be achieved in a single grinding operation; it is generally divided into several grinding passes, completed through a repetitive process of offset trial grinding – measurement – adjustment of the offset – and further trial grinding. Once the defect has been ground away, the part is re-aligned, and normal grinding is performed to correct the ovality. This method is relatively inefficient and requires operators to possess a high level of technical skill; however, in single-machine, single-piece production, it offers a high success rate and does not delay production schedules. Its effectiveness has been verified through several years of practical application.

II. Heat Treatment Expansion Method

The quenched microstructure of imported bearing steel consists of quenched martensite, a small amount of undissolved secondary carbides, and approximately 12%–14% retained austenite. Quenched martensite and retained austenite are unstable microstructures; during tempering, the decomposition of martensite causes the steel to contract, whilst the decomposition of retained austenite causes it to expand.

As the tempering temperature increases, the amount of residual austenite transformed and decomposed increases. Provided the required hardness is maintained, appropriately raising the tempering temperature to decompose the residual austenite and transform it into a martensitic structure with a higher specific volume can cause the workpiece volume to increase correspondingly, thereby relatively increasing the amount of outer diameter grinding required. By utilising this method, parts with defects in the imported bearing rings that would otherwise be scrapped under normal grinding conditions can be salvaged.

 This method is particularly effective for thick, heavy workpieces with a high residual austenite content (especially for self-aligning roller bearings). In actual production, different tempering processes are formulated for parts of varying specifications, dimensions and thicknesses. Under the premise of ensuring hardness and deformation, this allows for a thorough microstructural transformation, producing significant expansion and correspondingly increasing the grinding allowance, thereby salvaging scrap parts.

III. Chemical Deposition Method

For imported bearing components where dimensional tolerances exceed design standards, the chemical deposition method may be employed for salvage. The principle of chemical deposition involves chemical reactions between various raw materials to uniformly deposit a metal coating of a specific thickness onto the component surface; subsequent tempering ensures that the metal layer possesses the same hardness and mechanical properties as the original component. As chemical deposition merely increases the part’s dimensions without altering its geometric tolerances, it serves as an effective salvage method for parts with dimensional deviations. Currently, the thickness of a single-sided deposit can reach approximately 0.1 mm.